Fever (Pyrexia) or elevated body temperature above 99°F (37.2°C) is usually a deliberate resetting of the body's thermostat (in the hypothalamus of the brain) to fight infection, as it creates a less favourable environment for pathogens and enhances immune cell efficiency.
Fever may also be due to inflammatory, neoplastic (cancerous), and other causes. Fever is considered low grade up to 39°C (102.2°F), moderate till 40°C (104°F), and high till 41.5°C (106.7°F). Hyperpyrexia is an extremely high fever, exceeding 41.5°C (106.7°F) due to a malfunction in the hypothalamic thermoregulation, often associated with sepsis or central nervous system (CNS) infections or drug reactions that can lead to multiorgan failure and death if not immediately treated. Hyperthermia is an uncontrolled rise in body temperature (e.g., heatstroke) without a change in the hypothalamic thermostat setting. An acute fever is of less than 7 days, a subacute fever is of 7 to 14 days, and a fever lasting more than 14 days is a chronic fever.
NORMAL BODY TEMPERATURE The normal body temperature is 98.6°F (37°C), but varies from 97°F (36.1°C) to 99°F (37.2°C). • Normal temperature varies with age, time of day (lowest in the morning, higher in the afternoon), activity levels, measurement location, and equipment. • Infants and young children have slightly higher temperatures, while older adults have lower normal body temperatures. • Measurement location affects accuracy. Armpit is the least accurate, forehead or temporal is fast but affected by sweat or clothing, under the tongue or in the ear is more accurate, and rectal is the most accurate. Rectal is best for infants, while ear or oral is generally used for older children and adults.
BODY TEMPERATURE MEASURING INSTRUMENTS: • Digital thermometers use electronic heat sensors to provide fast readings (1-60 seconds) for oral, rectal, or underarm (axillary) use. • Infrared (Non-Contact/Forehead) thermometers use infrared technology to measure temperature from the forehead, ideal for quick, hygienic, and non-invasive checks. • Tympanic thermometers measure heat emitted from the eardrum, providing nearly instant results. • Temporal artery thermometers measure the heat from the temporal artery on the forehead. • Smart/digital sensors offer Bluetooth connectivity for tracking temperature over time.
TYPES OF FEVER: • Intermittent fever: Temperature increases off and on and returns to normal at least once a day, as seen in classic "spiking" fever of Malaria, abscesses (localized pus collection), and septic fever. • Remittent fever: Temperature fluctuates more than 1°C or 2°F per day but never touches normal, as seen with Infective Endocarditis, viral Infections, bronchopneumonia, Brucellosis, etc. • Relapsing Fever: Long, multi-day fever separated by days or weeks of normal temperature, as seen in Borrelia Infections transmitted by ticks or lice, rat-bite fever, Hodgkin's Lymphoma (Pel-Ebstein fever), Dengue, and some other viral Infections. • Continuous (Sustained) Fever: Temperature remains constantly high with minimal fluctuation, e.g., Typhoid fever, pneumonia, urinary tract infections. • Persistent low-grade fever of over 3 weeks, associated with autoimmune diseases or chronic infections like TB. • Inverse fever: A rare fever that peaks in the morning and decreases by evening, reversing the normal circadian rhythm. It indicates an abnormal temperature regulation often associated with infections like tuberculosis or typhus.
INFECTIOUS FEVER Infections are the most common cause of fever, occurring when pathogens like bacteria, viruses, fungi, or parasites trigger an immune response. • Viral: Common cold, influenza, COVID-19, Dengue, Chikungunya, Zika, measles, chickenpox, rubella, gut virus infections (rotavirus, norovirus), and infectious mononucleosis, etc. • Bacterial: Pneumonia, urinary tract infections (UTIs), strep throat infection, meningitis, and tuberculosis (TB), etc. • Parasitic: Malaria, leishmaniasis, toxoplasmosis, and various helminthic infections like schistosomiasis. • Fungal: Invasive candidiasis, histoplasmosis, coccidioidomycosis, blastomycosis, aspergillosis, and Pneumocystis pneumonia (PJP). Infections often present with "localizing" signs that point to a specific organ system, such as: • Respiratory: Cough, runny nose, sore throat, or shortness of breath. • Urinary: Painful or frequent urination. • Gastrointestinal: Diarrhoea, vomiting, or stomach cramps. • Systemic: Severe headache, stiff neck, or sensitivity to light (may indicate meningitis). Key Diagnostic Tests: • Complete Blood Count (CBC) to check for high white blood cell counts and inflammatory markers. • Blood, urine, pus, or throat swab culture to identify causative bacteria or fungi and their sensitivity to drugs available against them. • Chest X-Ray to detect pneumonia or lung infections, other tests of stools, spinal fluid, etc, depending on the infection suspected. • Specific Antigen or PCR tests are used for Malaria, Dengue, COVID-19, or Typhoid (Widal test).
INFLAMMATORY & AUTOIMMUNE CAUSES These often involve persistent, low-grade fevers accompanied by chronic fatigue, joint pain, swelling, and unexplained skin rashes. • These occur when the immune system mistakenly attacks the body's own tissues or responds to noninfectious inflammation. • Autoimmune disorders like Systemic lupus erythematosus (SLE), rheumatoid arthritis, vasculitis, or Still's disease. • Autoinflammatory genetic conditions like Familial Mediterranean Fever (FMF) and TRAPS. • Systemic Inflammatory diseases like sarcoidosis, inflammatory bowel disease (Crohn's, ulcerative colitis), and Kawasaki disease. Key Diagnostic Tests: • Inflammatory Markers: High ESR (Erythrocyte Sedimentation Rate) and CRP (C-Reactive Protein). • Autoantibody Panels: Tests like ANA (Antinuclear Antibody) or Rheumatoid Factor (RF) to detect specific immune system attacks. • Imaging: Ultrasound, MRI, CT, or PET scans to look for internal inflammation in organs or joints.
NEOPLASTIC (CANCEROUS) CAUSES Certain types of cancer can cause fever, often referred to as "malignant fever," by releasing cytokines or through tissue necrosis. It is often a diagnosis of exclusion after infections have been ruled out. It is often accompanied by night sweats, significant unintended weight loss, and enlarged lymph nodes. Causes include: • Haematological malignancies like leukaemia and lymphoma (especially Hodgkin's). • Solid tumours like renal cell carcinoma, pancreatic cancer, and hepatoma. Key Diagnostic Tests: • Naproxen Test: If the fever drops substantially after taking naproxen (an NSAID), it may suggest a neoplastic cause. • Imaging & Biopsy: PET, CT, or MRI scans to locate tumours, followed by a biopsy of tissue or bone marrow for confirmation. • Blood tests for specific tumour markers like LDH or Alpha-fetoprotein (AFP), etc.
MISCELLANEOUS CAUSES Environmental, metabolic, and chemical triggers of fever require checking environmental exposures or metabolic health. • Drug fever disappears shortly after stopping a specific medication, like antibiotics, anti-seizure drugs, or toxic or illegal substances (e.g., cocaine). Environmental factors like heatstroke, heat exhaustion, and severe sunburn cause high fever following heat exposure. Heatstroke presents with dry, hot, red skin and confusion, agitation, or unconsciousness, and a rapid, strong pulse requiring immediate shift to a cool area, water or ice packs application to cool the body temperature, and emergency treatment. Heat exhaustion is caused by water and salt loss, following heavy sweating, and presents with pale, cold, and clammy skin and a fast, weak pulse, requiring immediate cooling and hydration. • Endocrine disorders like hyperthyroidism (thyroid storm) and adrenal crisis. • Tissue damage due to major trauma, surgery, blood clots (DVT or pulmonary embolism), or brain haemorrhage.
FEVER/PYREXIA OF UNKNOWN ORIGIN (FUO/PUO) Fever of Unknown Origin (FUO) is defined as a fever lasting over three weeks without a specific cause identified despite extensive evaluation. Travel history, medication, occupational exposure, and pets handled are enquired about. PET-CT scans may help locate occult infection or inflammation. • Non-specific symptoms like chills, sweating, muscle aches, headaches, and weakness may accompany the fever. • If the patient is not critically ill, use of broad-spectrum antibiotics or steroids is best avoided, as these can mask symptoms and delay finding the true cause. • Elderly patients or those who are immunocompromised require more caution, as the risk of a hidden, serious infection or malignancy is higher. • Some undiagnosed fevers might be "functional hyperthermia" linked to psychological stress or sympathetic hyperactivity. • Even when the fever persists for a long time, most patients eventually recover without a specific diagnosis, without developing a severe, hidden disease.
FEVER CAN BE IGNORED OR MANAGED AT HOME IF: • If the person is alert, hydrated, and symptoms are mild, the fever itself is not dangerous and may help with recovery. • Children over 1 year who are responsive, playful, drinking fluids, with fever under 102°F (38.9°C). • Adults with a temperature below 103°F (39.4°C), lasting less than 72 hours, and accompanied only by mild symptoms (runny nose, cough, body aches). • Manage with rest, hydration, light clothing, and optional fever-reducing drugs (antipyretics) if uncomfortable. Allowing fever to run its course, within safe limits, can aid recovery.
FEVER NEEDS MEDICAL ATTENTION IF: • Infants less than 3 months with a fever of 100.4°F (38°C) or higher, having trouble breathing, stiff neck, purple spots or severe rash, persistent vomiting or diarrhoea, or signs of dehydration (no tears when crying, dry mouth, reduced urine). • Infants 3-12 months with a fever ≥102°F (38.9°C) or lasting >1 day. • Children with fever if dehydrated, lethargic, inconsolable, or having fits/seizures • Children and adults with a fever more than 103°F (39.4°C), or any fever with severe headache, stiff neck, chest pain, confusion, persistent vomiting, or difficulty breathing. • Anyone with weakened immunity, chronic illness, or recent surgery should not ignore fever. Fever may be serious, or even life-threatening, requiring urgent medical care, whereas mild cases can be ignored if quickly corrected or managed at home.
Dr. P.S.Venkatesh Rao is a Consultant Surgeon, Former Faculty CMC (Vellore), AIIMS (New Delhi), and a polymath in Bengaluru, drpsvrao.com.